1. Field of the Invention
The invention relates to a machining apparatus for super-finishing an outer peripheral surface of a rotating tapered roller.
2. Description of the Related Art
A tapered roller for use as a rolling element for a rolling bearing is produced by shaping through grinding and then super-finishing an outer peripheral surface of the tapered roller, which serves as a rolling surface. As an apparatus used for the super-finishing, a through-feed machining apparatus has been known (see, for example, FIG. 1 in Japanese Patent Application Publication No. 2002-86341 (JP 2002-86341 A). This machining apparatus includes a pair of drums on which a plurality of tapered rollers is mounted in juxtaposition. With the tapered rollers fed on and along the rotating drums, the outer peripheral surfaces of the tapered rollers are super-finished by a grinding stone.
When workpieces such as rolling elements (tapered rollers) for rolling bearings are mass-produced, the above-described through-feed machining apparatus, which has actually demonstrated high performance, enables the workpieces to be efficiently super-finished. However, when a large variety of workpieces to be machined (tapered rollers) are produced in small lots, the through-feed machining apparatus is unsuitable for the production. This is partly because different drums are needed for the respective tapered rollers. In other words, each time the size of the tapered rollers is changed, the drums need to be changed and adjusted. However, the drums are long in its axial direction and heavy, and thus, the adjustment operation is difficult and takes long time.
For the through-feed machining apparatus, when surfaces of the drums are worn away with a long-term use, the surfaces need to be machined. In some through-feed machining apparatuses such as the one depicted in FIG. 9 and including a pair of drums 90 and 90, a spiral groove 92 is formed in each of the drums 90 and 90 in order to rotationally feed tapered rollers 91. In this case, when worn away with a long-term use, the grooves 92 need to be machined. Machining the grooves 92 needs a dedicated grinding machine, and disadvantageously, maintenance of the drums 90 is difficult.
When a large variety of tapered rollers are produced in small lots, an in-feed machining apparatus is preferably used instead of the through-feed machining apparatus. The in-feed machining apparatus includes a pair of rollers. A single tapered roller is mounted on the pair of rollers, which is then rotated to rotate the tapered roller. A grinding stone is brought into contact with an outer peripheral surface of the tapered roller. Thus, super-finishing is performed on the outer peripheral surface. When this machining is ended, the machined tapered roller is unloaded from the machining apparatus. The next tapered roller is loaded on the pair of rollers, and super-finishing is performed on the tapered roller.
In the in-feed machining apparatus as described above, while the tapered roller is rotating stably on the pair of rollers, the grinding stone correctly contacts the outer peripheral surface of the tapered roller to achieve super-finishing. However, when the rotating tapered roller performs an unstable behavior, the grinding stone may damage the outer peripheral surface of the tapered roller. Specifically, when a (sudden) slip occurs between the pair of rollers and the tapered roller, the grinding stone may damage the outer peripheral surface of the tapered roller.